AS: accessory spleen

CT: computed tomography

MRI: magnetic resonance imaging

Retroperitoneal masses usually have a various spectrum of etiologies. ¹ Primary retroperitoneal ones are uncommon and divided into neoplastic and no neoplastic masses which are infrequent. ² They represent 15 to 20% of all primary retroperitoneal masses. ²

Accessory spleens (AS) are defined as a congenital defect with an additional splenic tissue to the native spleen due to an incomplete fusion of splenic masses during the embryologic period. ³ They are located elsewhere than a normal anatomical situation. ⁴ However, an ectopic spleen located other than in the left upper quadrant is an unusual situation. ⁴ The retroperitoneal AS is a rare entity. It is usually misdiagnosed, and mistaken for a retroperitoneal neoplasm. ⁵ Imaging findings are useful to characterize primary retroperitoneal tumors and to help the diagnosis of accessory spleen. Therefore, preoperative diagnosis is essential to avoid unnecessary surgery. ⁵

Our work highlights some of the diagnostic difficulties, describing a retroperitoneal AS removed on the suspicion of a retroperitoneal neoplasm considering that a preoperative pathological diagnosis could not be obtained.

We report the case of a 41-year-old woman, with no past medical or family history, who presented in August 2020 with an abdominal mass discovered by auto palpation associated with a left lower quadrant pain. No other abnormalities were found in clinical examination of the abdomen. The results of routine biochemical and hematologic tests were normal. Computed tomography (CT) scan of the abdomen obtained after intravenous administration of iodine-containing contrast material showed a 73 × 57 × 37 mm heterogeneous and partially necrotic retroperitoneal mass located along the left primitive iliac vessels ( Figure 1). The arterial supply of the mass came directly from the aorta. Multiple lymph nodes were identified around the mass and along mesenteric, aortic, and left primitive iliac vessels. The spleen was normal. We suspected a mesenchymal tissue tumor so that we decided to biopsy the mass. Ultrasound-guided fine-needle aspiration was done concluding hemorrhagic aspirate. T2-weighted magnetic resonance imaging (MRI) examination showed a left retroperitoneal heterogeneous and necrotic mass, surrounding the iliac vessels extended from the aortic to the left iliac bifurcation, iso-signal T1 and T2, hyper vascularized at the arterial time with a peripheral capsule intensely enhanced. The mass was vascularized by a supply artery native to the left primary iliac artery ( Figures 2a, 2b & 2c) and there were two lateroaortic vascularized masses of 35 × 16 and 29 × 25 mm ( Figures 3a, 3b). Abdominal organs were unremarkable.

Since a retroperitoneal neoplasm of unknown origin had been recognized, imaging findings had suspected a vascular tumor: hemangiopericytoma or paraganglioma. The embolization of the arterial supply was judged risky since the coils could move into the iliac vessel and cause ischemia. After consent, the patient underwent laparotomy resection of the retroperitoneal mass in October 2020. A midline incision was made. After exploration of the abdomen, we found a hard solid 10 cm mass left to the aorta, repressing medially the left colon ( Figure 4). There were no other abnormalities. We initially made a dissection and control on Suspensor Lake of left ureter and gonadic vessels in the retroperitoneum ( Figure 5a). The left primitive iliac, external iliac and hypogastric arteries were also dissected and controlled so that we could ligate the supplying vessels on the left primitive iliac artery ( Figure 5b). The dissection and control of the abdominal aorta permitted the ligation of an arterial supply with the mass and a para-aortic lymphadenectomy ( Figure 5c). Finally, a total resection of the retroperitoneal mass was made ( Figures 6, 7 & 8).

Gross examination of the resected specimen revealed a firm, encapsulated brown beige mass. Microscopic examination showed normal splenic parenchyma, no cell atypia, and no malignant cells ( Figure 9). Immunohistochemical study showed that CD20 emphasizes white pulp, CD30 marked intensely red pulp, and CD23 marked the dendritic follicular cells. There was no malignancy on the resected lymph nodes. The post-operative outcome was uneventful, and the patient was monitored regularly.

Primary retroperitoneal tumors are uncommon with a large range of malignant or benign histological forms. ⁶ There is a high rate of malignancy in these tumors so that the definitive diagnostic confirmation can only be made by the examination of the surgical specimens. ⁶

The ectopic spleen must be differentiated from the accessory spleen, which is a common congenital abnormality defined by the presence of normal splenic tissue located in an abnormal situation, in addition to a normally placed spleen. ^{3 – 5 , 7} AS have normal splenic histology and result from the failure of splenic cell fusion during the embryologic period. ⁸ Its incidence varies from 10 % to 30%. ^{5 , 9} Besides, an automatic implant of normal functional splenic tissue located in any compartment of the body outside the usual position can be classically seen. This entity is called splenosis and is mostly caused by abdominal trauma or surgery. ⁹

As reported in the literature, AS are measured generally around 1 cm in diameter. ^{5 , 7} Masses larger than 4 cm are rare. ³ They could reach 3–4 cm in cases of splenectomy response to compensatory hypertrophy or an associated hematological pathology. ^{6 , 8} However, in our case, the bulky mass exceeded this interval despite no history of splenectomy or any hematological anomaly. The AS is typically small, vascularized by branches of the splenic artery, ¹⁰ and is usually located near the splenic helium or the pancreatic tail. ^{4 , 5 , 9} However, one of the highlights of our case is the rarity of its location and vascular supply. The AS may mimic various tumors referring to its location such as a pancreatic tumor, adnexal tumor, retroperitoneal tumor. ⁷ In the retroperitoneal situation, most of them are located on the left side except three cases reported by Maharaj et al, Kim et al, and Zhou et al, in which they were situated on the right retroperitoneal area. ^{5 , 6 , 8}

These spleens are usually asymptomatic, but they can be complicated by torsion, rupture, hemorrhage, or cyst formation. ³ Representing a benign etiology, the diagnosis should basically be made preoperatively. ⁸ However, knowing that malignant retroperitoneal tumors are more common than non-malignant ones, the differentiation between these tumors and the identification of an accessory spleen can only be made after surgery. ⁸

Retroperitoneal masses are diagnosed and characterized by referring to the enhancement pattern using radiological findings: Ultrasonography, CT scan, and MRI. ^{1 , 7} But the diagnosis of the retroperitoneal accessory spleen is difficult and challenging on imaging. ^{8 , 9} Radiological imaging can mislead the diagnosis of the accessory spleen to retroperitoneal tumors. ⁴ CT scan findings demonstrate in typical cases, a mass with good margins similar to the splenic parenchyma in contrast. ³ MRI can also be used to evaluate tissue characteristics and accessory splenic pedicles. ³ In our case, imaging findings of the retroperitoneal mass led to misdiagnosis of the accessory spleen. They showed that the principal supply artery of the lesion was native of the left common iliac artery instead of the splenic artery according to the literature. ¹¹ On the other hand, the size of the mass in our case was much larger than reported in the literature, and its enhancement scanning pattern was different from the main spleen. Besides, percutaneous preoperative biopsy of the lesion was not sufficient to differentiate accessory spleen from other tumors, and it is not necessary when a soft tissue tumor is suspected. ⁸

Nowadays, only nuclear medicine imaging can confirm the diagnosis using Technetium-99m heat-damaged red blood cell scintigraphy or Tc-99m heat-damaged autologous red blood cell SPECT-CT, which are the key means to differentiate accessory splenic tissue. ^{3 , 11 – 13} This modality determines approximately the site of the accessory spleen, but it is only performed when there is a high suspicious index. ^{13 , 14} So if there is any diagnostic doubt or difficulty based on the routine imaging modalities, especially in patients with the previous splenectomy, preoperative nuclear medicine scintigraphy could be investigated to exclude the presence of an AS or splenosis. ¹⁴

In summary, when a difficulty is detected in diagnosing a retroperitoneal mass, only surgery can make a definitive diagnosis following the pathological examination of the specimen. ^{5 , 14} In our work, according to the results of contrast-enhanced CT and MRI, we suspected hemangiopericytoma or paraganglioma. The challenge posed is that retroperitoneum can include a large variety of tumors. In our case, MRI findings suggested a vascular tumor since the mass was intimately surrounding vascular structures so that hemangiopericytoma was the etiology considered preoperatively in our patient. It is a rare vascular tumor native from the pericytes of Zimmerman and an unusual retroperitoneal mass that requires surgical exploration and excision to establish the final diagnosis of the accessory spleen after pathologic examination. ¹⁵

The accessory spleen is usually misdiagnosed even using biological and imaging features and its management is challenging moreover controversial. ^{8 , 10} When symptomatic or following a complication, such as torsion of its pedicle or its rupture or suspected malignant transformation, surgical resection is necessary. ⁷ Surgical removal of an accessory spleen is also required when it is identified as splenectomy for the hematologic problem. ⁶

According to the literature, retroperitoneal accessory spleens are rare, and less than thirty cases were reported and published until 2013. ^{3 , 7 , 14} A review of the literature was performed using the PubMed Database. We used “retroperitoneal”, “accessory spleen”, “retroperitoneal tumor” as keywords for the research. After excluding articles that did not report AS, and those who reported ectopic spleen or splenosis, and manuscripts not written in English, we found 38 cases of retroperitoneal accessory spleen between 1969 and 2020.

The analyzed articles included four cases of spleens larger than 7 cm, as in our case. According to the literature, this dimension is unusual, and these accessory spleens usually do not surpass 4 cm in diameter. The diagnosis was possible preoperatively in one case using technetium-99m-labeled heat-damaged red blood cell scintigraphy. This is the only case where this technique was performed. All the patients underwent a CT scan for diagnosis. MRI was used in two cases only. The differential diagnoses were various, and all cases were diagnosed accessory spleen postoperatively, after the pathological examination. As in our case, the literature highlights the challenge of reaching a diagnosis before surgery. So, when a retroperitoneal neoplasm is detected, surgeons should be conscious of a possible retroperitoneal accessory spleen. Table 1 shows the challenging course to reach the final diagnosis which leads to surgical resection in all cases.

A wide spectrum of rare pathologies, including benign and malignant tumors, can be located in the retroperitoneum. The initial challenge in retroperitoneal diseases remains in the establishment of a diagnosis. Complete surgical resection is the only potential diagnostic modality. A retroperitoneal accessory spleen is extremely uncommon. The main misdiagnosis is a neoplastic disease so that the accessory spleen could be wrongly removed. Our case highlights the possibility of a retroperitoneal accessory spleen when investigating a retroperitoneal mass.

All data underlying the results are available as part of the article and no additional source data are required.

Written informed consent for publication of their clinical details and clinical images was obtained from the patient.